fix(share_plus): only configure popover presentation on iPad

[m1roxx]

Description

Since Xcode 26, UIActivityViewController.popoverPresentationController is non-nil on iPhone as well, where it returns a private _UIActivityViewControllerPresentationController instead of a real UIPopoverPresentationController. Logged on iOS 26.0:

iPhone: isIpad=0  popover=<_UIActivityViewControllerPresentationController: 0x102715c50>
iPad:   isIpad=1  popover=<UIPopoverPresentationController: 0x1017486d0>

FPPSharePlusPlugin set sourceView unconditionally and, on iPhone with a non-empty sharePositionOrigin, also set sourceRect through the else if branch. Both configure anchored presentation on a controller that is not a popover controller on iPhone.

This PR restricts every popover configuration to iPad, which is what the surrounding comment already stated as the intent. iPad behaviour, including the centre-of-view fallback added in #3769, is unchanged.

Verification

• iPhone 17 Pro / iOS 26.0, with and without sharePositionOrigin: share sheet presents as before and share() resolves.
• iPad Pro 11-inch (M4) / iOS 26.0: popover anchors correctly, centre-of-view fallback still applies when no origin is given, no exception.
• flutter analyze reports no issues.

I could not reproduce the hang described in #3943 on iOS 26.0 — 3/3 calls resolved, including a sub-screen round trip and an interrupted interactive dismissal. That report is against iOS 26.3–26.5.2, which needs a newer Xcode than I have available. So this change is verified non-regressive on both idioms, but confirmation that it resolves the hang itself needs someone running 26.3+.

Related Issues

• Fix [share_plus] iOS: sourceView set unconditionally on iPhone — on iOS 26 swipe-dismissing the sheet can hang the share() Future and break touch delivery #3943

Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I titled the PR using Conventional Commits.
• I did not modify the CHANGELOG.md nor the plugin version in pubspec.yaml files.
• All existing and new tests are passing.
• The analyzer (flutter analyze) does not report any problems on my PR.

Breaking Change

• Yes, this is a breaking change.
• No, this is not a breaking change.